Final answer:
Harold Pitcairn promoted the autogiro after acquiring the patent rights in the U.S. He improved the design, resulting in the Pitcairn PCA-2, and lobbied for its commercial use to ensure its popularity and adoption in aviation.
Step-by-step explanation:
Harold Pitcairn played a crucial role in promoting the autogiro, which is a precursor to the modern helicopter. Although he did not invent the autogiro — that credit goes to Spanish engineer Juan de la Cierva — Pitcairn made significant contributions to its development and popularity. Answering the question, Harold Pitcairn wanted to promote the autogiro, and thus the correct response would be (d) Promoted it.
After obtaining the patent rights for the autogiro in the United States, Pitcairn and his company, Pitcairn Aircraft Company, made several developments to improve the design and performance of these aircraft. His efforts led to the Pitcairn PCA-2, an autogiro model that became the first rotary-wing aircraft to achieve type certification in the United States. Pitcairn's efforts in advancing the autogiro included lobbying for air regulations that enabled commercial use of the autogiro, conducting public demonstrations, and using them in mail and goods delivery, which further helped in promoting the unique rotary-wing aircraft.